At present, there are two types of optical fibers, namely, single-mode and multi-mode optical fibers used in the field of laser processing by using a fiber laser. In a single-mode optical fiber used in laser processing, it is generally considered that a value of M2 as an index of beam quality is 2 or less. Therefore, since the beam quality of the single-mode optical fiber used to propagate emitted light of the fiber laser is better than that of the multi-mode optical fiber, the processing merit is great. However, an output power and a propagatable distance are limited by a nonlinear optical phenomenon, particularly, stimulated Raman scattering (SRS), and, for example, in the case of propagating a light wave with 1 kW or more, the propagatable distance is limited to several meters. For this reason, in some cases, the output power of the fiber laser may be propagated from several tens of meters to several hundreds of meters by using a multi-mode optical fiber and may be used for laser processing in some cases. However, in multi-mode fiber lasers, the beam quality and the value of M2 as an index of the beam quality are inevitably degraded in comparison with single-mode fiber lasers.
In addition, as disclosed in Non Patent Literature 1, a single-mode fiber laser capable of obtaining an output power of about 10 kW is realized, however the length of the delivery fiber (feeding fiber) is limited to several meters. This is because, in order to suppress the SRS with the single-mode optical fiber, it is necessary to enlarge an effective area (Aeff) as described later, but at the same time, since the expansion is a tradeoff with the increase of a bending loss, it is necessary to expand an outer diameter of the delivery optical fiber to the order of millimeters to realize a much larger Aeff. If the outer diameter is increased in this manner, flexibility of the optical fiber is lost, it is difficult to bend the optical fiber, and thus, there is a problem that damage or breakage easily occurs and the optical fiber is not suitable for long-distance delivery.
To summarize the above, in the related art, it is very difficult to propagate kW-class high power light with a single-mode optical fiber and a fiber laser from several tens of meters to several hundreds of meters.
In order to solve this problem, it is effective to enlarge the effective area Aeff (which is generally proportional to the square of a mode field diameter) which is one of parameters of the optical fiber, and for example, design of a single-mode optical fiber with various refractive index distributions disclosed in Non Patent Literatures 2 to 5 has been studied.
However, although the values of Aeff in the optical fiber disclosed in the cited literatures are prescribed, a propagation distance usable with respect to the output power and the like are not clarified, and it is not obvious whether the kW-class high power light can be propagated from several tens to several hundreds of meters. Furthermore, a range of the outer diameter of the optical fiber considering the flow of the fiber design and the practicality are not clarified.